The use of inkjet printing systems has grown dramatically in recent years, which is attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Notwithstanding their recent success, intensive research and development efforts continue toward improving inkjet print quality, while further lowering cost to the consumer.
With inkjet printing, a desired printed image is formed when a precise pattern of dots is ejected from a drop-generating device, known as a printhead, onto a print medium. The printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an inkjet printhead substrate. The inkjet printhead substrate incorporates an array of firing chambers that receive inkjet ink through fluid communication with one or more ink reservoirs. The inkjet ink typically includes one or more colorants dissolved or dispersed in an aqueous-based ink vehicle. Each firing chamber has a resistor element, known as a firing resistor, located opposite the nozzle so that the inkjet ink collects between the firing resistor and the nozzle. Each resistor element is typically a pad of a resistive material and measures about 35 μm×35 μm. The printhead is held and protected by an outer packaging referred to as a print cartridge or an inkjet pen.
Upon energizing of a particular resistor element, a droplet of inkjet ink is expelled through the nozzle toward the print medium. The firing of the inkjet ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, forming alphanumeric and other characters on the print medium. The small scale of the nozzles, which are typically 10 μm to 40 μm in diameter, require that the inkjet ink not clog the nozzles.
However, repeated firings of the resistor elements, which are designed to withstand millions of firings over the life of the print cartridge, result in fouling of the resistor elements with residue and degradation of pen performance. This build up of residue is known as kogation. The term “kogation” is used herein to refer to the build up of the residue, or koga, on a surface of the resistor element in the inkjet pen. Kogation reduces drop velocity and drop weight and reduces the efficiency of drop ejection. A loss of drop weight over the life of the inkjet pen reduces the chroma or optical density of the inkjet ink on the print medium and, therefore, degrades print quality. A loss of drop weight over the life of the inkjet pen reduces the accuracy of drop placement on the print medium and, therefore, degrades print quality.
Oxyanion additives, such as phosphates, in the inkjet ink have been disclosed to reduce kogation. The oxyanion additive eliminates or reduces adsorption of dye and/or decomposition products onto the resistor element. Short-chain phosphate esters, anionic phosphate ester surfactants, organic acid sulfonate additives (such as sodium methane sulfonate), and bile salt additives (e.g., sodium cholate) have also been disclosed for kogation control. It has also been disclosed that phytic acid in the inkjet ink reduces foreign matter deposits on a surface of an inkjet heating head. Although many solutions for kogation have been, proposed, many of the solutions are limited in their effectiveness, are not economically feasible, or have undesirable side effects for inkjet pens that need long resistor life.
Although kogation typically occurs with dye-based inkjet inks, kogation is also a problem with fixer fluids. As used herein, the term “fixer fluid” refers to a fluid that is substantially devoid of color and includes a reactive component (e.g., a molecule, complex, or a functional group in a molecule, polymer, or complex) that reacts with a component of the inkjet ink. In other words, the fixer fluid includes no colorant, such as a dye or a pigment, or includes a colorant that does not absorb visible light but absorbs in either or both of the infrared (“IR”) or (“ultraviolet”) UV spectrums.
The reactive component of the fixer fluid typically includes a cationic polymer, which is dissolved in an ink vehicle. When the fixer fluid is printed with an inkjet ink that includes an anionic dye, a printed image having increased waterfastness, smearfastness, smudgefastness or bleed alleviation, improved color vibrancy, improved edge acuity, or reduced dry time is produced. When the fixer fluid and the inkjet ink combine on the print medium, a precipitation reaction occurs which makes the dye durable and waterfast. The precipitation reaction also enhances optical density on certain types of print media, such as coated offset print media.
Many of the additives that have been used to improve the kogation of inkjet inks are not compatible with fixer fluids because the additives are anionic and, therefore, may precipitate with the cationic polymers. The precipitation may be even more pronounced when the additives are anionic surfactants that are incorporated into micelles because their anionic charge becomes concentrated. Therefore, it would be desirable to provide additives that are compatible with the cationic polymers in the fixer fluid to minimize kogation.